The main problem with using cpio is that individual files have a limit
of 2GB (which isn't a problem in most cases, but it is a limit). Why
not push a newer cpio format that changes all 16-bit and 32-bit
records to 64-bit (along with specifying endienness)?

What exactly is the use case of files larger than 2GB wrt RPM?
I just do not see the need for this.

Seems like computer history has taught us that this is a bad reason to
not extend something.

Exactly! Did you ever wonder what unix would have been like if it only
supported the operations where Ken Thompson had been given a use case?

"Rule of Extensibility: Design for the future, because it will be here
sooner than you think"